Sound at frequencies below 20 Hz is termed “infrasound.” A particularly favorable property of infrasound is its propagation over long distances with little attenuation. Infrasound has this property because atmospheric absorption is practically negligible at infrasonic frequencies, and because there is an acoustic “ceiling” in the stratosphere where a positive gradient of the speed of sound versus altitude causes reflections of infrasonic rays back to Earth. Infrasound propagation over long distances (e.g., thousands of kilometers) is predominantly due to refractive ducting from the upper layers in the atmosphere, while propagation over short distances is completed by direct path.
Infrasound is radiated by a variety of geophysical processes including earthquakes, severe weather events, volcanic activity, ocean waves, avalanches, turbulence aloft, and meteors. Infrasound sensors have been used for Nuclear Test Ban Treaty monitoring, for which there are a number of infrasonic listening stations throughout the world to detect large explosions and missile launches at a great distance. The National Oceanic and Atmospheric Administration has experimented with the use of infrasound for severe weather detection. Infrasound has been used to detect the occurrence of noise from a building implosion at a distance of twenty-five miles. It is likely that infrasound may be usable to detect and identify certain distant incidents/events of interest, such as objects dropping in water, explosions, wakes of aircraft and missiles, boats moving slowly, and vehicle and personnel movement associated with urban warfare. It is also likely that infrasound may be usable to detect natural events, such as clear air turbulence, distant forest fires, volcanic eruptions, meteors, tornadoes, landslides, and hurricanes.
Some weather-related natural events may currently be detected using electromagnetic (EM) detection systems, such as radar. However, such use of EM sensors has several drawbacks. First, EM sensors are unable to receive signal returns in clear air, where reflective targets (e.g., precipitation, particulate matter) are absent. Second, EM systems, being active, require scanning to locate an event. Third, EM systems, even weather radar (e.g., Doppler radar), have a limited range thereby requiring a large number of individual radar stations to provide detection over a large area. For example, the U.S. National Weather Service's Next-Generation Radar (NEXRAD) system uses 158 radar stations located across the U.S. to provide adequate detection.
Received infrasound signals are typically of low intensity (i.e., weak), and, as such, infrasound detection systems require highly sensitive microphones. A microphone is an acoustic transducer which produces an electrical signal as a result of a time-varying pressure in the air immediately in front of the microphone membrane. Several different types of microphones are available, with each type of microphone having a distinct transduction mechanism. In condenser and electret microphones, the transduction mechanism is based upon changes in the stored electric field energy. In a condenser microphone (also termed an air-condenser microphone), acoustic energy causes small movement of the microphone diaphragm (also termed a membrane), which serves as one plate of a parallel-plate capacitor. The condenser microphones are high-impedance devices with amplifiers located near the sensor itself. These microphones are stable with temperature and environmental changes because of stainless steel diaphragm. A condenser microphone requires a high DC voltage between the membrane and backplate, called the “polarization voltage.” The polarization voltage is typically 200 volts for linear operation. In an air condenser microphone, the polarization voltage is applied from an external source. In an electret condenser microphone, the polarization voltage is applied by means of a thin layer of electret material which is deposited on the backplate and subsequently polarized.
Referring now to FIG. 1, a cutaway view of a conventional air condenser microphone is illustrated. In a condenser microphone, an incident sound pressure excites motion of a stretched membrane or diaphragm. The motion of the membrane changes the capacitance between the membrane and backplate, thereby producing a proportional output voltage. Hence, the performance of this type of microphone depends upon an electrical as well as mechanical system of the microphone.
When a fixed charge is applied on the plates of the membrane-backplate capacitor through a large resistor, the motion of the membrane changes the voltage between the plates. The charge is maintained by a high voltage, called a “polarization” voltage. This technique has the advantage of very low thermal noise, thus providing very low threshold detectability (i.e., increased sensitivity). However, this technique suffers at low frequencies due to the finite charging time of the capacitor.
The function of the mechanical system of a condenser microphone is to provide damping of the membrane motion for an optimally flat microphone frequency response. The microphone operates at frequencies below the fundamental resonant frequency of the stretched membrane. At frequencies approaching the resonant frequency, the response shows a pronounced peak if the membrane is not properly dampened. As the membrane vibrates, it compresses and expands the air layer in the gap and creates a “reaction” pressure, which opposes the motion of the membrane. The reaction pressure generates airflow which introduces damping primarily at two places: in the gap between the membrane and the backplate, and in the openings (holes and slots) in the backplate. A sufficiently small gap may, by itself, provide the necessary membrane damping, but the necessary small size would conflict with the requirements of electrical and mechanical stability. The damping is thus augmented by the flow of air through the holes and slots in the backplate, which provide large surface areas for viscous boundary layer damping.
The backchamber serves as a reservoir for the airflow through the openings in the backplate. If the cartridge were perfectly sealed, then a constant quantity of air would remain within the microphone interior. A vibration in ambient pressure would result in a pressure differential across the membrane, a shift in the membrane's static position, and a change in microphone sensitivity. For this reason, a capillary vent hole is introduced to provide static pressure equalization on the two sides of the membrane. The capillary vent hole leads from the backchamber to outside of the microphone. However, the pressure equalization system, like the electrical charging system, causes the response to roll-off at low frequencies.
Referring now to FIG. 2, the basic equivalent circuit of a known polarized microphone is illustrated. In FIG. 2(a), the microphone is shown as a variable capacitor, Ct (because of diaphragm deflection due to time-varying outside pressure); Cs is the stray capacitance (due to lead capacitance); Ci is the input capacitance; Ri is the input resistance to a preamplifier; and Cc is a large blocking capacitor used to protect the preamplifier. To minimize the stray capacitance Cs, the preamplifier is located physically as close to the microphone cartridge as possible. The primary function of the preamplifier is to provide the low output impedance needed to drive the output cables and prevent loss of bandwidth due to cable capacitance, especially if the connecting cables are long. In FIG. 2(b), a well-regulated voltage source E0 establishes a charge Q and voltage E on Ct through resistor Rc, such that Q=CtE. As the membrane of microphone vibrates, the microphone generates a time-varying current It.
There are several properties of condenser microphones which reduce their suitability for the detection of infrasound. For example, condenser microphones generally always have a very high resistance Rc (on the order of 4 giga-ohms) inserted in series with polarized voltage as shown in FIG. 2(a). Also, condenser microphones have an input resistance Ri to the pre-amplifier. These two resistances (Rc and Ri) are both sources of Johnson noise, which elevates the noise floor of the microphone and thus reduces its sensitivity. Further, condenser microphones have a capillary vent. Also, a high polarization voltage has to be applied to such microphones. Finally, condenser microphones require an external polarization voltage (to be applied between the diaphragm and the backplate). Condenser microphones suffer at low frequencies due, at least in part, to the finite charging time of the capacitor.
Electret microphones (also termed electret-condenser microphones) are similar to condenser microphones, except electret microphones use a thin polymer film coated on the topside of the backplate. The film is polarized permanently at a level comparable to that used in condenser microphone. The linearity, frequency response, and transient response of electret microphones are similar to that of condenser microphones. The main advantage of the electret microphone is that it operates without the need for an external power supply, and is available at relatively low cost. In an electret microphone, the capacitor's charge is permanently embedded in a layer of electret material that forms the part of backplate. As it is unnecessary to apply a polarized voltage, the high resistance (adjacent the power supply) is eliminated; this makes the electret microphone more sensitive than a standard condenser microphone.
In addition to microphone sensitivity issues, infrasound signals are typically contaminated with wind noise and other background noise. As such, infrasound detection systems require some mechanism capable of screening wind and other noise from the infrasound signal. Known infrasound detection systems use a plurality of lengths of porous garden hose (often termed “soaker hose”) arrayed outward from an infrasound microphone. The porous hose serves as a low-frequency mechanical filter. Each segment of porous hose is typically laid in a shallow, open ditch to further reduce wind noise. Referring now to FIG. 3, a top view block diagram of a known infrasound detection system is illustrated. In the prior art system 10 of FIG. 3, the infrasound microphone 12 is located at the center of twelve sections of porous hose 14. Each section 14 is typically 250-400 feet long. Thus, the overall size of such a system is typically 250,000-640,000 square feet. The very large size of the prior art system severely limits the portability of this system and limits the number of possible locations at which the system may be deployed. The large size also severely limits the ability to arrange multiple systems into an array, which is necessary to determine the direction of an infrasound signal.
As such, there is a need for an infrasound detection and measurement system capable of detecting very low intensity infrasound signals, and further capable of doing so in a small, portable form.